US2894836A - Sintering of shaped cemented metal bodies - Google Patents

Description

July 14, 1959 F, K'QLBL ETAL I 2,894,836

SINTERING O SHAPED CEMENTED METAL BODIES Filed Nov. 19, 1957 HIII! o A IN1/ENT Rs 41u94 A. Mesa-Aff@ wwf M Mm United Se@ Parent O" v 2,894,836 d .Y smTERlNG or SHAPED CEMENTED METAL Booms Franz Kolbl and Karl Messmer, Reutte, Tirol, Austria, Vassignors to Schwarzkopf Development Corporation, New York, N.Y., a corporation of Maryland lApplicati-on November'19, 1957, Serial No. 697,449

. :s claims. (c1. VFls-zoo) This invention relates to the production of shaped cemented metal, particle bodies containing ingredients which at sintering temperature, form a liquid phase.

vrIn the past, Adiliculties have been encountered in producing shaped `cemented refractory metal particle bodies having ingredients of relatively low melting temperature, because of the 'deformationof the shape of such bodies which occurred in the course of the iinal sintering treatmentto which they have to be subjected in order to give thenrthe desired physical characteristics and strength. Thus,'by way of example, in the case of a small gas turbine rotor formed of titanium carbidebound by an alloy of nickel chromium and cobalt forming more than about 30% of the rotor, it was found that its shape becomes deformed when it is subjected to the inal sintering treatment at the required hightemperature, such as 1300 C. to 1350* C. (Throughout the specification and claims, all proportions are given by weight unless specically stated otherwise.)

Among -the objects of the invention is a sintering treatment for sintering cemented metal particle bodies containing ingredients which form a liquid phase at the sintering temperature, which treatment will suppress and eliminate the deformation of the desired shape of the body caused by the inalsintering treatments` as carried onin the past. Theforegoing and other objects of the invention will be best understood from the following description of exempliiications thereof, reference being had to the accompanying drawing, the single figure of which shows, by way lof example, in a partially diagrammatic and par tially cross-sectional view, one type of equipment for sintering a cemented refractory metal particle body in accordance with the principles of the invention.

- 'Ihe present invention is based on the discovery that the deformation dicnlties encountered in the iinal sintering, of ,cemented refractory metal vparticle bodies are caused by the differences in the gravitational forces' acting onthe different parts of the body as a result o'f partial softening of the lower melting phase thereof when heated during theiinal sintering treatment. In accordance -with the invention, the deformation di'iculties encountered in the nal sintering treatment of such cemented refractory metal bodies, areeliminated and suppressed by rotating the body during the sintering operation so that the gravirational forces acting on dilferent por-tions of the cemented bodybalance each other. For best results, the cemented body should beA rotated about one of its axes of inertia, which vare also the axes of symmetry of the body. Thus, in the case of a cemented body which is symmetric with respect to lan axisof rotation thereof, the body should be rotated around such axis of rotation during the sintering treatment. In the case of bodies which have in onedirection materially smaller dimensions than in other directions, such as disc-like bodies, it is desirable to rotate the body during thezsinteringroperation `about a horizontally of equipment which has been found satisfactory for carry-` duid-guiding buckets 14 symmetrically arranged with,

respect to the axis of rotation 12. Such gas turbine rotors, which have to operate in oxidizing combustion gas atmosf. pheres at eleva-ted temperatures, are produced out of cemented refractory particle material which exhibits the.

required strength and corrosion resistance at the elevated temperatures at which it is exposed to the combustion gas atmosphere. used to produce gas turbine rotors of the entire range of ysizes in which they are required. As an example, the

process of thev invention may be used for making gas .turbine rotors as small as about 5 centimeters or even less in diameter, as well as larger sizes such as upto 30 centimeters or even larger in diameter. r

As an example, vgood results are obtained with a gas turbine rotor made by the process of the invention out of titanium carbide particles cemented by a binder alloy metal of high corrosion resistance but having a lower meltingV temperature than titanium carbide.- As a more specic example, good results are obtained with such gas turbine rotor having 30% to 70% titanium carbide content, the balance binder metal of high corrosion resistance and good binder properties. Good results are obtained with the binder metal formed of nickel, cobalt and ohrarmium or nickel and chromium. For binder meta-l of nickel, chromium and cobalt, the content of nickel may vary from 50% to 80%, of chromium from 20% to 50%, of cobalt from 20% to 30%. Good results are obtained with a binder metal containing nickel, 20% chromium and 20% cobalt. With a binder metal consisting of nickel and chromium, good results are obtained with 80% nickel and 20% chromium.

By way of example, in practice, good results have been obtained with a gas :turbine rotor made with the process of the invention, out of such cemented material having a diameter of 25 centimeters and weighing 9 kilograms. Good results are also obtained with similar smaller and larger size rotors having a correspondingly smaller or larger weight.

Asa further example, similarly good results are obtained with a turbine .rotor formed out of ZrBg or TiB2 lwith 5% to 20% of CrB2 in solid solution therewith, constituting all its content or instead only 30% .to .70% of its content, and as balance either 50% to.80%r nickel, 20% to 50%.ch1'omium and 20% to 30% cobalt, or 50% to 80% nickel and 20% to 50% chromium, such as 80% nickel and 20% chromium.

The process of the invention will now be described in I more detail in connection with a specic example thereof.

disposed'inertiaaxis thereof corresponding to its largest 70 momentJof inertia.-V

p In thedrawingisshown by way of example, one type Thus, ,a cemented gas turbine rotorr is produced by the process of they invention as follows:

Example 1 l removing from the die, the green compact is, pre-sintered.

Patented July 14, 1959'` The process of the invention may be 3 in vacuum or in a protective latmosphere at a temperatureV between about 800 C. and 900 C. for three to four hours, depending on size, so as to yield la pre-sintered bodyyofz substantial strength but sufficient softness` to permitshaping by. conventionalitools to the desired final shape.. The. pre-sintering; temperature` is so chosen `as to` prevent.softening of the` shaped body while securing substantiallyV fullV shrinkage thereof, and, minimize Iany further shrinkage thereof in` the final sintering treatment. In. the. designing of the; dies for the green compact, allowance` is made for the shrinkage of the compact in the sintering. treatments andA for the shaping ofA the pre sinteredbody. The. machined and finished pre-sintered rotorA body so obtained `is thereafter subjected to the final; sintering treatment at an elevated temperature between abo1it..1300 C. to 1350.o C. for l to. 11/2 hours depending on the size.. By way of example, a gas turbine rotor ofz high desired thermal shock resistance` was. made `by the process described above out of` a powder mixure.` containing 35% titanium carbide. and 65% binder metal consisting of 60% nickel, 20% chromium; and 20% cobalt. The resulting rotor bodies had a.A transverse `rupture strength. of 180 to 200 ktg./mm.2 andv a1 specific weight of 6.9 g./cm.3.

i Therewillnow be described in connection with the drawing, how a gas turbine rotor of the type described above, was subjected to vthe nal sintcringoperation at a temperature of about 1300 C. to 1350 C. for about one hour or more, without causing it to be deformed, in accordance with the principles of the invention. The desired? shaped cemented bodies, such as turbine` rotors are subjected to the final sintering operation Within theinterior` of a furnace which, in the form shown, comprises a double-wall casing enclosure 21 formed of two spacedA walls 22, 23 of heat-resistant material, such. as heat-resistant metal, confining a cooling space through whichr a. cooling liquid, such -as water, is circulatedfor cooling the furnace-walls. The furnace enclosure 21 is shown provided with a detachable double-wall section 24, likewise formed of two spaced walls 22-1, 23-1 confining 4a cooling space through Which cooling liquid is. similarly circulated. The detachable enclosure wall section24 isrjoined to the main enclosure 21 along the intertting junction Walls 25, 26, arranged so `as -to permit joining them with a hermetic seal which permits maintenance with the interior enclosure space of the furnace either a vacuum or. a` desired protective atmosphere, such as hydrogen, during the sintering treatment. The tur bine rotor. 10.is shown held on a rotor shaft 31 arranged to be rotated,` on bearings 32. The rotor shaft is formed of refractory material. The left side of the rotor shaft 31A is shown arranged to be joined by detachable coupling connector 33 to a drive shaft 34 which is rotatably driven by a suitable drive mechanism onthe interior of the furnace enclosure 21. The rotor shaft 31` shown is` connected by` a similar coupling connector 33 to a similar rotor shaft of the adjacentl of a series of rightwardly disposed additional turbine rotors which are to be sintered within the furnace enclosure 21 in accordance with.the principles of the invention. It is to be understood-that the partof the specification below, which dea ls with-.the 'turbine rotor 10 mounted adjacent to the left side wall of the furnace enclosure 21, apply also to each of the other similar turbine rotors which are subjected simultaneously to a similar final sintering operation within the furnaceenclosure 21.

The drive shaftV 34 by which the rotor shaft 31 of the turbine rotorl is driven, `is shown seated for rotation in aabearing 35l mounted in the left side wall of the furnace enclosure 21. The bearing 3S is of a type well known inthe artrthat provides hermetic seal between the interiorf andthe exterior of thefurnace enclosure 21' while permitting the shaft 34 passing therethrough to rotate therein `for driving through coupling connector 33, the rotorsha-ft: 31 together with the turbine rotor `10l held CII thereon, andother similar turbine rotors which are being sintered within the furnace enclosure 21.

The region of each of the turbine rotors 10 positioned within the furnace enclosure 21 is sho'wn arranged to be heated by `an array of heater bodies shown in the form of heater rods 41 of a materialssuch as molybdenum or molybdenum disilicide, which are arranged so as to` be supplied with electric heating current as through electric connector bars 44 extending from Water cooled electric connector terminals 45` insulatingly mounted and( passing through the wall portions ofthefurnace enclosure 21. In order to` protect the interior Wall surfacesV of? the furnace enclosure 21 against` the heat radiated by` the heater bodies 41, a plurality of heater screens, shown L by dash lines 46 andiformedof sheets of refractory metal,

such as molybdenum, are interposed between the heater bodies 41 and the interior wall surfaces of the furnace enclosure 21. Inlorder tol maintain the sintered body 10 at adesired uniform` andbalanced treatmentftemperature, there is` provided within `the furnace enclosure 212', a further.` inner heat-balancingrenclosure: 51 enclosing and surrounding thelspace occupied by the rotor10.`

The. innen enclosure 511 may beiformedrof refractorymetal such4 as molybdenum sheetv material andmay have a detachable. wall section 52 on` the upper `sidef'offthe adjacentnregionof the detachable walla24 of' the furnace enclosure `21. In the Aform` shown, the bearings 212 on whichv the rotor shaft 31'- is suit-ablyA mounted Iwithin the walls of the inner enclosure 51 surrounding the cemented rotor 10IwhichV is to1 be= sintered, may be-formed` off re'- fractory ceramic material ofthe' type usedy in'- similar applications, `which will retain its strength at the highA temperatures at' 'whichA the interior of the furnace is' heated; 'Ihegeneral details of` thel structural features 0f' the sintering furnace of the type shown, except#` for the arrangement whereby the sinteredbody is tohbe rotated therein, are of- Wellrknown construction generally used inthe pasti for finally sintering cemented refractory metal particle bodiesv andA need not be described inY more detail. The rotol'fshaft 31 onwhich the cemented rotor 10= is. seated, the drive shaft 34- and the shaft' connector 33 are made of a solidmaterialV that willretain! its strengthrat the elevated@ furnaceytemperature, such as tungsten` or molybdenum.

Therinteriorof the sintering `furnace 2,1l vis-'heated by heating current suppliedl tothe heater bodies;41\ for bring ing andV maintaining therotor 10 at the desired sintering temperature in the range between aboutI 1300* C. to 1350io C. The interior space withinthe furnace enclosure 21 is evacuatedr by a suitable vacuum connection, not'shown,.so as toi maintain thereinay vacuum of about 0.1A to OLOS mm. of a mercury column. Throughout the time the interior of the furnace enclosure Z1 is heated. to a` raised temperature at which the binder metal of the rotor might develop a liquid phase or mightsoften, the rotor 10is rotated by applying arotary` drivingforcerto theouter end` of the drive shaft 34. The rotor10 isy kept rotating for the entireV periodlof 1 to2 hours required for completing the final sintering treatment wherein its body is given the desired` physical characteristics. After performing the sintering ytreatment-the heatingl is stopped and the interior of the` furnace is cooled, the rotorV` 10 beingrkept rotating untilitstemperature has been brought downconsiderably below the temperature at which the binding `metal'thereof hasany liquid*l phase or a-tendency tosoften, such as atemperature'of 8009 C.V orlower.

Inlmaking gasturbine rotors by a process of therinvem tionof the typerdescribed above, it has been found advantageous to incorporate smalliamounts of molybdenum inthe mixture of the. powder` ingredients` outof which the gas turbine rotor is formed. The addition ofa small amount of molybdenum. eliminates free: carbon and p oxygen which' if present would` detract' from the `physical ture of`600 C. to 900 C. for a pre-sintering time of about .l'hour and up Ito 3 or 4 hours, depending on the siz'e. The machined and finished pre-sintered rotor body is subjected to a final sintering temperature of 1200 C. to l400 C. for l to lll/z hours.

Very good results have been obtained with the process of the invention carried on within the range of conditions outlined above, with a gas turbine rotor having an outside diameter of 8 inches and larger.

In the sintering operations, during which the gas turbine rotor is subjected to elevated temperatures at which its binder may become softened, the gas turbine rotor is rotated around an axis of symmetry thereof to prevent ow of the softened or liqueed binder metal due to the gravitational forces acting thereon. In practice, slow rotation f the gas turbine rotor such as at l r.p.m. (revolutions per minute) or a somewhat slower or higher rate,

s uch as rpm., is sufficient to prevent gravitational ow or creep of the heated softened binder metal.A By such slow rotation, the direction of the gravitational 'forces acting on the softened binder metal is continuously changed atla rate at which they are insulicient to cause any movement or displacement of the softened binder metal of the rotonwhile it is heated during the sintering operations. By' rotating the rotor in the manner described above to prevent flow or creep of binder met-al While the rotor is subjected to the elevated ntemperature of the sintering furnace, runiform and homogeneous distribution of the binder metal is assured, and the danger of nonhomogeneity avoided, a critical factor for rotors whichV is"formed. Thus, in the case of-a gas turbine rotor of 35% TiC cemented by 65 %l of the binder metal, as described above, having an outside diameter of 20 centimeters, go'odresults are obtained by rotating the rotor at a'rat'e of about 4 to 5 r.p.m. while it is maintained at the elevated sintering temperaturej On the other hand, in the case of a similar gas turbine rotor of the same material' having an outside diameter of about 14 centimeters, good results are obtained by rotating it at a rate of 2 to 3'r.p.m. 'Rotors made with a smaller content' of the binder metal, may be rotated lat a smaller speed to prevent gravitationalffow or creep of the binder metal which is somewhat softened at the elevated sintering temperature. `In general, a relatively low speed of rotation, of at most 200 revolutions per minute, will be suicient for preventing deformation of .a cemented body as a result of softening of fa liquid phase thereof during the sintering operation.

The sintering method of the invention is. not limited to forming shaped cemented bodies of the type described above, but is of great value generally in the production of'all types of sintered cemented metal particle bodies containing ingredients which form a liquid phase at thev high sintering temperature to which they are subjected, because the action of the gravitational forces on the liquid phase of such body during the sintering may cause displacement of portions of the liquid phase, thereby disturbing the homogeneous character of the sintered body, a factor which is of great practical importance in many applications of sintered cemented refractory bodies. Thus, -for instance, lack of homogeneity in a rotary body will develop large unbalancing forces during high-speed rotation of such body and will require careful balance test vtoi ydetermine the radius of unbalance, followed b y.

careful removal of accurately determined. portions ofthe body for giving the body the -desired dynamic balanceQv Lack of homogeneity in the parts of a cemented body may lalso cause critical distortions thereof when subjected to heat shocks.

Among the applications in .which the sinteringftreatment of the invention is of great value, is the productiony of cemented particle tool elements, Y cemented-particle guides or nozzles for guiding or discharging liquid metal, cemented-particle sockets, and various other shaped cemented bodies. Additional examples of sintering treatments of the invention will be described below.

Example 2 A spiral heating element is made out of a powder mixture containing MoSiz particles and 10% CrB2, ground to a particle size of -50 microns.' The powder mixture having admixed thereto a plasticizing addition such as 2% of parane and having a dough-like consistency, is extruded in an extrusion press into an elongated strand. While still plastic, the strand is wound. on a. suitable cylindrical support into a spiral heater body having turns of circular shape, and dried on the support. After drying, the spiral is positioned on a sufpport of smaller' diameter than'its turns with theend portion of the spiral symmetrically held relatively vto the central support by end members of refractory material such'a's porcelain, seated on the'support and engaging the end turn of the spiral.

jected to a sintering treatment at about 1800 C. in* a hydrogen atmosphere for 15 minutes, while the spiral heater and its support are rotated at a rate of 5 revolutions per minute. Such spiral heater, when sintered without rotation while suspended vertically, would cause an-enlargement ofthe lower spiral turns. Such spiral heater whensintered without rotation while in a hori-A zontal position, would cause the circular spiral turns to be deformed into oval shape.

Example 3' ings, andkfor molds used for casting reactive metals,jfor

protecting thermocouples placed in liquid bodies such as molten aluminum, and in like applications. shaped article is sintered while it is Abeing rotated vat the rate ofl about 5 to -10 turns per minute in a sintering furnace. To permit rotation of the shaped article being sintered, it is formed with end portions along which-itmay be rotatively supported in suitablebearing supports within the sintering furnace. After completion of the, sintering treatment, the end portions are ground away, leaving the desired shaped body. Such shaped articles, when made without rotary sintering supports of the invention, .would undergo deformation under the action of gravitational forces while it is softened by carbide, 5% of tantalum carbide, 8.5% cobalt, and the balance tungsten carbide, is compacted into a thin cylinder,

the thickness of which is such that after shrinkage, it will have the thickness of the desired spiral tool elements which are to be secured on the cylindrical support sur- The support with the spiral are then sintered in a sintering furnace and sub-V 7 faceofthe` steelv rotor; After an initial presintering at about1800 C. to: 1000. C. for to' 10 minutes, the pre siriteredr` compact is subjected to a grooving operation for forming out of the cylindrical' sintered compact al f plurality of spirally-shaped elements joined and supported at the Vopposite ends by a continuous end ring collar. After' placing the so-obtained generally cylindrlcal body f on' a cylindrical graphite rod on which its end collars i Example 5 l For shield'containers for radioactive substances formed oftungsten bound with 5% nickel and 2% copper, it isy essentialtto secure absolutely uniform distribution and homogeneity ofthe ybinder metalV content throughout the body of the container. A cyli'ndrically` shaped'radiation shield is prepared' by` subjecting the compact formed of a lrnixtureofl several powder ingredients to sintering for 10Hto l5 minutes at l"400 C. to l450 C. undery hydrogen while the shield body is being rotated aty a ratev ofk live turnsl per minute; kDuring the sintering operation, the' body is supported on a cylindrical ceramic support which forms the rotating support thereof. the great difference between the specific weight of tungA sten' and its copper-nickel content, the gravitational forces acting thereon duringsintering, while itY contains a liquid phase, willl cause non-uniform distribution of its nickel'- copper content, if it is` not rotated duringA the sintering treatment' in accordance with' the invention.

Exampie 6 Air-foil shaped turbine` buckets. or vanes are formed out of titanium carbide cemented with 50% of a nickelchromium-cobalt alloy. The powder compact formed of the titanium carbide and the alloy, is provided at its opposite ends with rotaryA support `projections which are ground offtherefrom after completing the sintering. The rotary end portions of the compact are 4arranged coaxial with its `inertia axis. After preliminary sintering of the compact at about 800 C.-l000 C., it is subjected to anadditional shaping operation. Thereupon it is subjected toA additional sintering while it is being rotated on its end projections under vacuumV for 10 minutes at a sintering4 temperature of` 1250l300 C., at which a liquid phase is formed of some of its contents. Although suchl bucket is only 7` inches long, such sintering treatment willcause slight deformation thereof as a result of the gravitational forces acting thereon while it is softened? by the liquid phase formed therein during the sliort-` sintering treatment. The removal of the defects causedby the deformation involves expensive, careful machining, which is eliminated by rotation thereof during the sintering, in accordance with the invention.

Instead of providing a direct mechanical coupling conneetion between the exterior drive shaft and the rotor shaft 31, the mechanical driving connection may be provided in some other way, for instance by a magnetic clutch between a magnetic member of the external drive shaft and a magnetic member in the interior of the furnace enclosure 21 which has a mechanical connection to the rotor shaft 31.

Because of- This application is a continuation-impart of our coe: pending `application* Serial'No. 3813598', tiled September 2,2, 1953, now abandoned.`

The features and principles underlying the invention described above in` connection with specific expernplic :`2 1`` tions, will suggest to` those skilled' in the art manyother modifications thereof. It is accordingly desired that the appended ,claims be construed` broadly and that 'they shallnot belimited to thel specific details shown .and def scribed in connection with exemplitications` thereof.

We claim: v 1. The method of sintering a homogeneous cemented body consistingof cemented refractory particles selected from the group consisting of the refractory metals, the

carbides, the borides and the silicides` of refractory. me.-

tals and mixtures thereof, which body contains ingredientsV which form a liquid phase att-he sinteringternperature, the procedure comprising rotatably supporting said body along an axis, subjecting said body while so supported to sintering' for at least lO minutes at a sinteringA temperature of at least about 1100? C. at' which a liquid phase is formed of some of its ingredients tending to soften saidbody, and slowly rotating` said body about' said axis while it is being sintered at said sintering temf perature in such manner that the direction of, the gravitational forces acting on said liquid phase and other portions of said body is continuously changed at arate at.`

which said gravitational forces are insuliicient to cause any movement of portions of the liquid phase and'` of any other portions of said body relatively to` each' other be# cause ofsofteningof said liquid phase while being subjected to said sintering treatmen.

2,; The method of sintering a cemented body as claimed inclaim 1, wherein said body is rotated about an axis yhaving the greatest moment of inerti-a.

3. The method of sintering a homogeneousk cemented` f body which is symmetric with respect to a central axis` and consists of cemented refractory particles` selected from the group consisting of the refractory metals, the.

carbides, the borides and the silicidesof refractory metals and' mixtures thereof, which body contains ingredients which form a liquid phase at the sintering temperature, the procedure comprising rotatably supporting saidbody along 4said axis, subjecting said. body while so supported.

to sintering for at least l0 minutes at a sintering temperature of at least about 1100 C. atrwhich a liquid phase is formed of some of its ingredients tending to` soften said body, and slowly rotating said body about said central axiswhile it is being sintered at `said sinter ing temperature in such manner that, the direction ofthe gravitational forces acting on said liquid phase` and other portions of said body is continuously changed at a rate: at which said gravitational forces are insuicient to cause any movement of any portions of the liquid phase and of any other portions of said body relatively `to each other because of softening of said liquid phase. while being subjected to said sintering treatment.

4. The Vmethod of sintering a cemented body as claimed in claim 1, wherein` said body is rotated until" the temperature of said body is lowered from the elevated temperature of sintering to a level at which all binder metal content ofV said body is solidified.

5. The method of sintering a cemented body as claimed in claim 3, wherein said body is rotated -until the telnperature of said body is lowered fromthe elevated tem-` perature of sintering to a level at which all binder metall content of said body is solidified.

No references cited.

' MBL 3L,

UNITED STATES PATENT OFFICE ,f CAT 0F 0N Patent N00 2,891,836 v July 14, 1959 Frenz Kolbl et al.,

It is hereby certified that error veppeers in the above numbered patent requiring connection ,and that tne lseid Lettere Patent should read .as con@ nested belowo In thev heading to tbev printed specification, between lines "7 and 8, insert =m Claims priority,. application Austria Gctobe' 1, 1952 (SEAL) Attest:

ROBERT C. WATSON Attesting Officer comissioner of Patents UNITEOsTATEs PATENT OFFICE CERTIFICATE 0F CORRECTION N00 ,891,836 y l July 14, 1959 Fra-nz Koibi et an t iS hereby certified that error ,appeans in the above numbered patent requiring correction and. that the ,said Lettes Patent should read .as con rested belowo In the heading tol ther printed specification, between lines '7 and 8,

' insert m Claims priorityy application Austfia October l, 1952 Signed and sealed this 8th day of December 1959.,

(SEAL) KARL -10 Attest:

, ROBERT c. WATSON Atiiesli'ldlgl Officer Conmissioner of Patents

Claims (1)

1.THE MOTHOD OF SINTERING A HOMOGENEOUS CEMENTED BODY CONSISTING OF CEMENTED REFRACTORY PARTICLES SELECTED FROM THE GROUP CONSISTING OF THE REFRACTOR METALS, THE CARBIDES, THE BORIDES AND THE SILICIDES OF REFRACTORY METALS AND MIXTURES THEREOF, WHICH BODY CONTAINS INGREDIENTS WHICH FORM A LIQUID PHASE AT THE SINTERING TEMPERATURE, THE PROCEDURE COMPRISING ROTATABLY SUPPORTING SAID BODY ALONG AN AXIS, SUBJECTING SAID BODY WHILE SO SUPPORTED TO SINTERING FOR AT LEAST 10 MINUTES AT A SINTERING TEMPERATURE OF AT LEAST ABOUT 1100* C. AT WHICH A LIQUID PHASE IS FORMED OF SOME OF ITS INGREDIENTS TENDING TO SOFTEN SAID BODY, AND SLOWLY ROTATING SAID BODY ABOUT SAID AXIS WHILE IT IS BEING SINTERED AT SAID SINTERING TEMPERATURE IN SUCH MANNER THAT THE DIRECTION OF THE GRAVITATIONAL FORCES ACTING ON SAID LIQUID PHASE AND OTHER PORTIONS OF SAID BODY IS CONTINUOUSLY CHANGED AT A RATE AT WHICH SAID GRAVITATIONAL FORCES ARE INSUFFICIENT TO CAUSE ANY MOVEMENT OF PORTIONS OF THE LIQUID PHASE AND OF ANY OTHER PORTIONS OF SAID BODY RELATIVELY TO EACH OTHER BECAUSE OF SOFTENING OF SAID LIQUID PHASE WHILE BEING SUBJECTED TO SAID SINTERING TREATMENT.

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